The objective of this work has been to develop and characterize a mouse epidermal cell culture model system for studying promoter-dependent preneoplastic progression and its prevention. Recently published results have shown that JB6 mouse epidermal cells respond to second-stage but not first-stage (described by Slaga et al., Proc. Natl. Acad. Sci. USA, 77, 2251-2254, 3659-3663, 1981) tumor promoters to undergo irreversible promotion of anchorage-independent growth and tumorigenicity. promotion of tumor cell phenotype by phorbol esters is blocked by inhibitors of second-stage promotion such as retinoids but not by inhibitors of first-stage promotion such as antiproteases. The mechanism of promotion of tumor cell phenotype in JB6 cells appears to involve induction of a new phenotype(s) rather than selection of preexisting variants. Clonal derivatives of JB6 mouse epidermal cells which are promotable by phorbol esters are also promotable by other classes of tumor promoters such as ingenols, growth factors, detergents and cigarette smoke. Their nonpromotable counterparts have to date shown consistent cross-resistance to several classes of promoters suggesting that promotion sensitivity in JB6 cell lines is determined by a pathway common to a variety of promoters.